Locking fasteners are in widespread use and, particularly in the aerospace industry, must meet exacting performance standards. A common form of locking fastener, among several different types, uses a small plug of plastic material embedded in the wall of the bore of a nut for engagement with a bolt or stud that enters the nut. The plastic plug pushes the threaded shank to the opposite side of a nut causing the threads of the shank and of the nut to be pressed together at that location. The result is a frictional effect to prevent undesired loosening of the nut.
This system has its drawbacks, particularly in the area of repeatability. That is to say, the small plastic plug rapidly wears out upon repeated cycles of mating and unmating of the nut and the threaded shank, soon losing its ability to exert a substantial lateral force on the shank. The ability to lock the nut against loosening becomes lost. Then it becomes necessary to discard the nut, because it will not remain secured under vibrational loads.
Another form of locking nut has employed a ring of material around one end of the nut, but again the material has worn out rapidly and the locking effect soon vanishes. Other locking fasteners rely upon an out-of-round threaded bore, a resilient clamping force on the threaded shank or a distorted thread configuration. Adequate thread locking force, repeatability, and manufacturing cost have presented problems for these and other locking fasteners.